The adult fairy is non magical being as tall as a tennis ball with a wing span of a standard tennis racket. They are fairly agile in the air with maximum flight ceiling of 1000 m from ground level and they can fly up to 12 hours before needing to take a 1 hour break. A hundred fairies can easily lift a stretch of standard tennis court net over a distance of 100 m in 10 minutes. These fairies reside on top of Mt Everest and are adapted to the extreme cold and strong winds, the population hits 50000 with half of them combat ready.

A rumor is spreading like wildfire around the region that there is a dragon egg on the top of Mt Everest and the person who finds it shall be granted immortality by consuming it. The king wants this treasure and sends an army of soldiers to search the mountain. "All who oppose shall die in pieces" and the fairies fearing for their safety must protect their habitat and their young against the greedy humans. Contact between the two species is limited and the dragon egg does not exist.

How can the fairies stop a medieval 100000 strong army?

Note that the medieval soldiers are not well accustomed to the harsh condition at such high altitude; however, the men will stop at nothing to achieve the goal. Answer with the least casualties on both side wins. Just to clarify, the king is a bastard who only negotiates with the dead.

$\begingroup$Well, you do realise the inconsistencies of your question, right? They fly up to 1000m but live on the Everest (> 8000m). And I pretty much doubt you'll fit 50,000 tennis balls on the top of the everest, let alone have them develop a community. Since the top is already pretty packed, how are you bringing a 100,000 men-stronged army up there? I'd say your faeries, who live in a form of alternate dimension to accomodate their numbers are pretty safe: no such army will ever reach them.$\endgroup$
– bilbo_pingouinOct 27 '15 at 7:07

1

$\begingroup$Why would your soldiers not consume the egg themselves? Your evil human king has obviously not thought this through.$\endgroup$
– JDSweetBeatOct 27 '15 at 13:28

2

$\begingroup$So the faeries just have to take care of the few soldiers who survive and are suffering from extreme hypoxia? Even with modern equipment, Everest is a respected mountain, claiming more than enough lives each season. Is the king patient enough to wait for the safe season, and to pay attention to the 4-5 day "windows" where the mountain is safe to climb before the jet stream renders it inhospitable?$\endgroup$
– Cort AmmonOct 27 '15 at 21:46

$\begingroup$It seems there is only one problem in this whole scenario. The king needs eliminating.$\endgroup$
– KalleMPNov 12 '16 at 15:44

$\begingroup$@bilbo_pingouin: No inconsistency. They fly up to 1000m over local ground level, probably due to losing their navigation track if they fly higher.$\endgroup$
– JoshuaApr 1 '17 at 4:20

10 Answers
10

They just need to sit back and wait. The invading army will cripple itself from cold, avalanches and oxygen deprivation long before reaching the top of Everest.

The first two people to climb Everest without supplemental oxygen were Habeler and Messner in 1978. Messner later described the experience this way:

Breathing becomes such a strenuous business that we scarcely have strength left to go on. Every ten or fifteen steps, we collapse into the snow to rest, then crawl on again. My mind seems almost to have ceased to function. I simply go on climbing automatically. The fact that we are on Everest, the highest mountain in the world, is forgotten – nor does it register that we are climbing without oxygen apparatus.

With the invading army in that state the fairies need only trigger a few avalanches or just sneak in and start stabbing the people around the edges of the camp every time the invaders try to rest and before long there will not be much army left.

$\begingroup$The nightmarish words of the adventurous folk who have summited Everest and returned alive always astonish me, no matter how many times I read them.$\endgroup$
– Cort AmmonOct 27 '15 at 21:48

$\begingroup$Yeah, I can't imagine there being much need for the faeries to even worry at all. The idea of an army trudging up Mt. Everest successfully is somewhat laughable.$\endgroup$
– David RiceOct 28 '15 at 17:45

1

$\begingroup$Never mind trying to supply an army in the Himalayas... How did Hannibal do in the Alps?$\endgroup$
– wedstromApr 27 '16 at 18:55

1

$\begingroup$The fairies obviously started that dragon egg rumor, to get the stupid bastard king to deplete his army.$\endgroup$
– Nahshon pazSep 3 '17 at 7:12

Poison

Ignoring the inability of a medieval army to attack Everest (I'm assuming that Everest was an exaggeration and the actual mountain is feasible), your fairies would have to use poison to defend themselves. Four humans per fairy means that the fairies have to be more effective individually than humans. Presumably a medieval army could produce decent helmets to protect faces. So unless the fairies are ridiculously fast, the humans could swat them out of the sky.

The fairies need to be extremely effective with their attacks and they can't rely on precise targeting of vulnerable areas. The only real option in pitched battle would be poison. If the fairy can get within contact range, he or she applies a small amount of concentrated poison. This should be within their weight bearing capacity. A sudden snipe attack from an odd direction could be devastating.

Target the rope

Assuming that there is climbing involved, the humans will be using a lot of rope. Cut it; fray it; burn it. Occasionally this will precipitate a fall. Hopefully the falls will be as dangerous to the humans as attacking is for the fairies.

Concentrate your forces

It's hard to climb too close, so the humans will be spread across the face of the mountain. The fairies are smaller and should be able to concentrate their forces. So the poison carrying fairies will harass those at the top and the bottom while the burning/cutting fairies will concentrate on the rope. Even though the fairies are outnumbered overall, they can still concentrate more (of their smaller troops) at any particular location.

Attacking while a human is bringing up the first rope or shortly after the initial success would be important. If there are too many people there, they can defend the top of the rope better. If there is only one, then the fairies can release the rope and force the humans to send another climber.

Use the distance

The fairies should start attacking at the bottom, but they shouldn't worry too much about initial success. It's not a real problem for the humans to get half way up the mountain. So be most careful when the humans are farthest away. Look for opportunities to inflict casualties with low risk.

Note that the humans can afford to lose four soldiers for every fairy warrior. Make sure that your attacks are more effective than that.

The more spread out the human soldiers are, the easier it will be to attack effectively. The fairies should pace themselves. Maintain the constant threat of attack so that the humans need to waste energy defending against them. But limit most of the actual attacks to the most vulnerable sites. A single climber with no defenders above is most vulnerable.

Use sabotage to start fires during night attacks. So the humans have to waste time bringing up fuel for visibility. Cut the climbing ropes at night to slow the troops. Look for that vulnerable member who just wandered into the bushes to take a leak. Fake an attack one place to draw defenders from the place where you'll actually attack.

Many attacks will be more annoying than damaging. That's all right. Annoyed people take dangerous short cuts that leave them vulnerable. Also, the distance means that slowing the army down is useful in and of itself. A mountain is hard to supply. The more people devoted to supplies and defending against night attacks, the fewer actually climbing the mountain.

Assassinate the king

If the biggest obstacle to peace is the king being unreasonable, get rid of the king. This would be the method with the fewest casualties (conceivably as low as one). Of course, they may not realize that it's the king that's the problem. Also, an actual attack on the king may involve the fairies taking heavy casualties.

Give the king what he wants. Construct a fake egg, and send out a small party of fairies to 'coincidentally' meet the brunt of the king's forces, and be captured/killed.
To make it more believable, convince the fairies carrying out the mission that the egg is real, and they need to move to a safer location through the prepared route. Have them try to protect the egg with their lives, and if they are convincing enough, the king may believe that it is the real egg.

Get an ostrich egg and decorate or something. If the king is stupid enough to send 100000 soldiers to Mt. Everest on a rumour, he will be stupid enough to believe the fricking fake egg is the one he is looking for. And he can't really know for sure it's not real, unless he dies.

The faeries best bet is to use the mountain itself against their enemies. Specifically, they should use the snow that's covering the mountain. With the armies of their enemies marching up to get them they should do as much as possible to destabilize the snow without actually triggering an avalanche prematurely. When their enemies arrive, a small disturbance will be all they need to send hundreds of tons of snow, rock, and ice hurtling towards the approaching hordes at a lethal velocity. A big avalanche can easily scour an entire valley of approaching forces, or wipe them off of any narrow shoulders they try to cross.

Much of the approaching army will be killed in the avalanche itself, either crushing them, suffocating them, or hurling them off cliffs, but even for those left alive, it will wipe out any fixed lines or ladders that they've used to climb up. The survivors will be isolated, without supplies or knowledge of the mountains, and probably unable to retreat from the mountain the way they all approached. Being on Everest will be enough to finish them off.

Leave.

I'm assuming the fairies can't just make a fake dragon egg and give it to the king.

One can theoretically see around 208 miles from the top of Mount Everest. If we ignore the added height flying fairies would get, this is the earliest warning they would have of an invasion. We can cut that distance in half so the fairies can be absolutely certain it's an army (and also to give a fairy time to look in the right direction). This gives them some time to plan and prepare.

Unfortunately, given their isolation, the fairies probably don't know what the army is for. You don't give a flight speed for your fairies, but going off the lifting data each fairy can fly (carrying 1/100 of a tennis net) about 4.5 miles in a 12-hour period before requiring rest. If we assume fairies don't like to camp out, then the army will be a little over 2 miles away from Mt. Everest by the time they're close enough for a fairy to visit them and fly back without rest.

Assuming the humans and fairies speak a common language, it seems like a fairy would quickly discover why the humans had come, probably by being questioned at the point of a sword about "the dragon egg of immortality". Since he doesn't know, relations would ... quickly sour. Luckily, due to his relatively small size and flying skill, dodging arrows and the like should be pretty easy. There's a potential subplot in here about rescuing other fairies who set out when the army was 4.5 miles away and were captured while resting, which could be used even if the fairies don't end up fleeing, but I digress.

It's surprisingly difficult to find a source on the weight of tennis nets, but this listing says one net weighs 20 pounds (there's also a 'championship' version that weighs 22 pounds, but I'll stick to 20 for the purposes of round numbers and skewing numbers against the fairies). If 100 fairies can easily carry 20 pounds, that gives the entire population of 50,000 fairies a total lifting capability of 10,000 pounds, or 5 tons.

Tennis balls are around 2 inches in diameter (and I'm assuming fairies can fold up their wings, or have architecture that's flatter and wider than human housing to leave space for the wings). In any case, they would comfortably fit into a 1:24 scale dollhouse. Again, it's difficult to find a weight for a dollhouse, but most sources seem to indicate a full-sized house at this scale is around 10 pounds. These are the ones meant for serious collectors, so they include furniture and presumably are made out of sturdy wood (I was also able to find dollhouses that were 2 pounds, but those were flimsy).

If we assume 4 fairies can live comfortably in a single dollhouse equivalent, that's 12,500 fairy households. That's a total weight of 125,000 pounds, or 12 and a half trips for the fairies. They can relocate to pretty much any mountain peak, but the closer they settle the more time they'll have to carry things from their old home. Let's assume they choose a spot 12 hour's flight away. This would take them 14 days to do, including rest and the flight back. On the last trip, since only half the fairies are needed, the other fairies could perform some of the other actions suggested here to slow the advance of the army if necessary. In fact, it should be possible to split the difference between all required trips so that a group of fairies is constantly harassing the army. Obviously, this should be done such that nobody has any idea it isn't just bad luck, since the king wouldn't take kindly to sabotage.

Assuming the army takes 20 days to reach the summit (a gross exaggeration), the fairies still have enough time to make 5 more trips, carrying an additional 25 tons of assorted food, medicine, seeds, livestock, and/or pets to their new home, where they can wait for the king to get tired of searching around in the oxygen-low, freezing cold mountain for a dragon egg that doesn't exist.

And in all likelihood, the fairies don't have nearly that much stuff, their new home would be much closer, and the army would take way longer to scale the mountain.

The faeries can fly and are small enough to be easily missed. This means they can easily sneak anywhere. Combine that with tricks like poison and they can kill pretty much anyone in their sleep.

Guerrilla raids that involve slipping in, poisoning things, and leaving would be the appropriate tactic. They could poison the food and water supply, or simply knock it over and let it spill over the mountain. They can ensure that the leader of any military dies in his sleep. They can also have humans so paranoid of suddenly being poisoned in their sleep that they will be too afraid to sleep, leaving them all exhausted and jumpy.

Meanwhile you can set up all kinds of traps for them to fall into. They don't know this mountain or its cold weather, which puts them at a significant disadvantage, physically and psychologically. Look how how powerful Germany was simply by retreating into their winters and starving people to death. The pixies can do better, since they have drastically increased mobility (flight) they can set up traps and then flee without much risk of being caught and being forced to fight. Even if each trap is only slightly effective, they can set up as many as they want without much risk. Their enemy doesn't know the area, so they will not know what to watch out for.

If you combine this with a terror approach you have a pretty effective strategy. You don't need to defeat the majority of the humans, just make them not want to attack. If you can make them afraid enough, while eliminating their leadership, destroying their supplies, and preventing them from sleeping at night, it won't be long until they regular soldiers simply revolt rather them keep marching into a death they don't know how to deal with.

The fact that you have home ground advantage and can afford lots of low-risk runs through the time that they are marching helps. They're in very cold high mountain region they don't know how to live in and are vulnerable, your first few attacks and traps can force them to move slow, which buys you more nights to harass them.

Keep them thinking you have more numbers as well and are ready to fight with poison in an all out fight when they reach their destination and...yeah they will turn around and go home rather then face such an unknown.

From the information you give on the abilities of the fairies, there really isn't much of a chance for them to engage physically. If it takes 100 of them to even move a tennis court net, and even then it takes them 10 minutes to drag it for 100 meters. From that let's make a calculation:

Suppose a tennis net weighs about 20kg(I have no idea of the real one, so I just pulled this number from my sleeve). To keep it airborne against gravity takes F*m newtons, so 9.81 * 20 = 196,2 Newtons. So one fairy in this scenario exerts about 2 newtons of force. From this abstract, the force required to pierce the skin with a needle is between 0.1 and 3N, depending on needle. So theoretically the fairies might have just enough force behind them to actually pierce the skin, if they are able to craft needle-sharp weapons.

The amount of work they can exert is still ridiculously tiny to consider them doing pretty much anything in the ways of physical traps, so the only shot they have is the ability to fly and poke the soldiers to death. Seeing as the medieval soldier would likely be clad in leather armor, or at the very least some thick wool, the only chance of any realistic damage is to go for the face.

Your description of these fairies and their aerial capabilities in terms of agility and finesse is too general to even start guessing at their chances of doing high speed poke-by's for the eyes, but that's the thing I'd bank my money on.

The fairies have an air force that can fly up to 3,000 feet above the ground and carry things. The mountain territory will likely force significant parts of the army to coalesce, for instance to pass through a gorge. At this time, the fairies (using their knowledge of the terrain and several flying scouts to predict when this will happen) can launch a large troop loaded with poison gas or pepper gas. This will either kill or incapacitate the human army. Some of the fairies can instead carry pots of highly flammable liquid and drop pots of coals afterward. The pots should be ceramic so they release their content on impact. Other fairies can carry crossbows or longbows.

Medieval technology and a conquest of Everest are rather incompatible really (they don't call it the deathzone for nothing), but I'd suggest rocks, assuming that that's 1000m over mean elevation. An object dropped 1000m is going 140m/s (that's about 300 miles an hour) when it hits the ground, which is almost accurate at that altitude, air resistance is negligible that high up. At that speed half a kg (approx 1 pound) has roughly the same kinetic energy as a 12 gauge solid slug shotgun round. Hell even a ball of ice is going to be pretty lethal from on high.

If fairies do know about incoming army (by means scouts, travellers, few strange ones listening to the lives of people below mountain or anything plausible) and people do have means and will to climb on the mountain, faeries have a quite a few ways to attack.

nature, as mentioned before by avalanches, cold and lack of oxygen

natural aerial, faeries may use shaped water containers and move it higher where it'll freeze and will form a spike. Carried by bomber teams (to increase load-out mass, even 1 pound should be enough) and dropped even from 20-80 meters above the target it will have enough destructive force to stun/shock brain/fracture skull (even protected by helmet) or injure body of any man that will be hit by it. It will also provide quite good weapon of terror especially when some crazy fairies will try dive bombing with some sort of battle cry like "…iiiiiIIII!!". Saying so it may be very hard on their bodies, but in general population you will always find these kinds of troublemakers who will merrily like to try it.
Double effective when performed during sleep as people usually do not sleep in full armor not mentioning that such an armor had to have a high price and is really heavy and makes people cold.
What's less funny fairies with their numbers may pour water (in manner of carpet bombing) on humans. It will be hard to soak thick clothing and fur in one-go, however faeries may come back. If by some luck water will create ice frozen solid shell on human it may effectively save him/her from further soaking. But ice also have own weight and human feels soo tired…

sneak attacks, forget about assassinations with fancy blades and poison, it will be enough if faeries will soak human clothing with water, especially the one they will wear in their sleep easily freezing their limbs, faces. This way they can easily even freeze human to death at first try, but chances are the targets will quickly wake up as cold water will reach their skin and try to do something about it (if only they have still enough strength left).

poison, applied to food supplies by stealthy fairies probably during some diversion (like head sized ice razors killing animals or falling on people). Leaving poisoned needles in places human will reach and not see.
Ultimately poison may be applied by assassination, but it should aim only sleeping or lone targets to reduce risk.
Eventually poison may be released from some containers as deadly vapor, probably heavier than air to further increase it's effect on climbing humans.

terror and atrocity warfare, it would be essentially force of faeries, mostly covered by unstoppable and unforgiving Nature and elusive deadly wraiths (fairies). May be expanded by usage of psychoactive poison.
Not mentioning it's hard enough to bring all this food and warm cloth up. Fairies may decide it wouldn't be strange for some more accidents to occur.

Dragon. They say there is no smoke without a fire. It may happen that there really was an egg of the dragon. By the time rumor got to the humans and spread (with the speed of messenger or some other riders) and the great army was formed AND got to the fairies neighborhood, dragon had more than enough time to grow and mature. Whether fairies succeeded with recruiting it for their cause the may bring it to the battle, where it's potential depends on your world. Eventually, Dragon far ago could fled, leaving the equation.

I would argue on effectiveness of flammables as cold environment of high mountain would make it harder to actually ignite (except for the Dragon, flying flamethrower). Military devices or some special tools would make it possible to burn despite environment, however it would require fairies to be more advanced than humans and probably heavily militarized.
On the advantages, water boiling temperature goes down with higher altitudes making it easier to actually boil humans alive.

Finally, as it was mentioned by Nepalese for Ant they may just leave if they don't have any special reason to stay (e.g. memory/will of their great ancestors, sacred mountain or pride).